1. Field of the Invention
The present invention relates to an optical writing head driving device and a method of driving the optical writing head which optically writes data onto a photosensitive member with a plurality of light-emitting diodes.
2. Description of the Related Art
Forming an electrostatic latent image onto a photosensitive member with lights from an optical writing head comprising a plurality of light-emitting diodes (LED) in an image forming apparatus such as a printer has been known as a conventional technique. In such a technique, for example, the LEDs are arranged in a line in the primary scanning direction, that is, in a direction perpendicular to the sheet feeding direction. The LEDs emit lights in accordance with image data and optically write the image data onto a photosensitive member.
Generally, the LEDs in the optical writing head emit lights unevenly because luminescence of each of the diodes differs from each other. The unevenness of the lights from the LEDs causes deterioration of an electrostatic latent image to be formed on the photosensitive member, and a resultant image on a sheet is also deteriorated.
To overcome the above problem, a modified optical writing head driving device can adjust unevenness of lights from LEDs in an optical writing head. FIG. 11 is a schematic diagram exemplifying such a conventional optical writing head driving device whose LEDs emit lights evenly.
An optical writing head driving device 100 shown in FIG. 11 comprises a ROM 101, a head controller 102, and an LED head 103.
The ROM 101 is an EEPROM or the like with small capacity, and inputs and outputs data serially. The ROM 101 previously stores adjustment data for adjusting unevenness of lights from LEDs in the LED head 103.
The head controller 102 reads out image data from a frame memory or the like (not shown), reads out the adjustment data from the ROM 101, and outputs the read image data and adjustment data to the LED head 103.
The LED head 103 comprises a plurality of LEDs arranged in a line in the primary scanning direction and a driver circuit which drives the LEDs to emit lights. The LED head 103 adjusts drive signals from the head controller 102. The drive signals correspond to the image data. The LEDs are driven in accordance with the adjusted drive signals and emit lights.
An operation of the optical writing head driving device 100 will now be described with reference to a timing chart shown in FIG. 12. The head controller 102 outputs a chip-select signal (CS) to the ROM 101, as a result, the ROM 101 is selected. The head controller 102 outputs a sync signal (SCK), a command for reading and address (Si) to the selected ROM 101. The head controller 102 serially reads out an 8-bit adjustment data (So) from the ROM 101 while being synchronized with the sync signal (SCK). The head controller 102 outputs the read adjustment data and the image data from the frame memory (not shown) to the LED head 103. The LED head 103 adjusts the drive signals from the head controller 102. The drive signals are prepared for driving the LEDs and correspond to the image data. The LEDs are driven in accordance with the adjusted drive signals and emit lights for optically writing image data onto the photosensitive member.
As described above, the optical writing head driving device 100 has to take steps of reading the adjustment data from the ROM 101 by the head controller 102 and outputting the adjustment data from the head controller 102 to the LED head 103, in order to make the LEDs emit lights evenly. Because of those required steps, it takes a longer time for optical writing in such a device than in a device without a light adjusting function.
Time (TW) for reading the adjustment data (So) for one line where the ROM 101 is an EEPROM whose clock frequency is 2 MHz and the number of the LEDs arranged in the LED head 103 in a line is 3840, is obtained by an equation of: EQU TW=1/2(MHz).times.8(bit/dot).times.3840(dots).apprxeq.16(msec.)
Velocity (Vppc) of the optical writing process where the resolution is 300 dpi (dots per inch: 1 inch=25.4 mm) is determined by the adjustment data reading time (TW) for one line with an equation of: EQU Vppc=25.4(mm)/300(dpi).div.TW.apprxeq.5.3(mm/sec.)
A printer or the like having the optical writing head driving device 100 as its printing section cannot realize high speed printing because its throughput is a couple of sheets per minutes when using sheets whose length in the sub scanning direction (i.e. the sheet feeding direction) is 210 mm.